The invention relates to an improvement in the Andrussow process for making HCN. In particular, the invention relates to improvement of the efficiency of the process.
Hydrogen cyanide (HCN) is one of the smaller volume industrial chemicals, which nevertheless is quite important in the chemical industry. In particular, HCN is used for the manufacture of cyanuric chloride, methyl methacrylate, adiponitrile (for nylon-6,6), sodium cyanide, ferrocyanides and chelating agents.
At present, almost all of the world's production of HCN is made by one of three processes:
(1) The Andrussow process in which ammonia, methane and oxygen are reacted over an oxidation catalyst to form HCN, CO, water and H.sub.2 ;
(2) The "methane-ammonia direct process" or Degussa process in which ammonia and methane are reacted in the absence of air in externally heated tubes containing platinum/rhodium catalyst to form HCN and hydrogen; and
(3) The Shawinigan process in which ammonia and propane are passed between spaced electrodes within a fluidized bed of coke. In addition to the above processes for making HCN directly, it is also made as a byproduct in the manufacture of acrylonitrile by reaction of propylene and ammonia over an oxidation catalyst.
Though each of these processes is used commercially, by far the most widely used is the Andrussow process. In the Andrussow process, a vapor phase mixture of oxygen-containing gas (usually air), ammonia (NH.sub.3) and methane is contacted with platinum metal catalyst at a temperature of about 1200.degree. C by which part of the methane is burned to furnish heat to the methane-ammonia reaction, which is endothermic. The overall reaction of the process is as follows: ##EQU1## Other byproducts of the reaction are H.sub.2, CO, CO.sub.2 and N.sub.2.
One characteristic of the Andrussow process is that the catalyst becomes less active with use. Though the cause for such deactivation is not precisely known, it is believed to be in part due to the formation of carbon on the catalyst, which results in a blocking of part of the active sites on the catalyst surface with a thin layer of carbon. In order to avoid this, it has heretofore been necessary to operate with lower concentrations of CH.sub.4 in the reactant feed to the process. Consequently, efficiency of CH.sub.4 utilization is lowered.
Because of the substantially higher cost of natural gas, which is the primary source of methane, and the shortage of natural gas, especially during periods of high residential use for heating, it is essential that available methane sources be utilized in the most effective manner.